Introduce new volume dialog.

 - New VolumeDialog (presentation) + VolumeDialogController (state)
   to implement a volume dialog that keeps track of multiple audio
   streams, including all remote streams.
 - The dialog starts out with a single stream, with more detail available
   behind an expand chevron.
 - Existing zen options reorganized under a master switch bar
   named "Block interruptions", with "None" renamed to "No interruptions"
   and "Priority" renamed to "Priority only".
 - Combined "Block interruptions" icon replaces the now-obsolete star/no-smoking
   icons in the status bar.
 - New icons for all sliders.
 - All sliders present a continuous facade, mapped to discrete integer units
   under the hood.
 - All interesting volume events and state changes piped through one central
   helper for future routing.
 - VolumePanel is obsolete, still accessible via a sysprop if needed.
   Complete removal / garbage collection deferred until all needed
   functionality is ported over.

Bug: 19260237
Change-Id: I6689de3e4d14ae666d3e8da302cc9da2d4d77b9b